The present invention relates to a position controller for a motor which performs position control based on a loading position signal from a position sensor attached to a load which is driven by the motor.
Priorly, in a motor controller for driving a direct-acting mechanism by means of a ball screw (hi-lead screw), etc., in general, an angular velocity of a motor is fed back to construct a speed control loop and an angle of the motor is fed back to construct a position control loop. In this case, if the motor comprises only an angle sensor such as a rotary encoder or the like, a position signal of the sensor is subjected to a finite difference operation so as to become an angular velocity signal. Hereinafter, such a control system is referred to as a semi-close control system.
On the other hand, in order to control a direct-acting mechanism with high accuracy, in some cases, a straight-line motion position sensing means such as a liner scale or the like is attached to a movable table of the mechanism, whereby constructing a position control system by means of an output from the sensing means. Hereinafter, such a control system is referred to as a full-close control system.
A block diagram of such a full-close control system is as shown in FIG. 13.
In FIG. 13, 701 denotes a position control portion and the position control gain is Kp. 702 denotes a speed-control portion, 703 denotes a motor, and 704 denotes a load (a machine movable portion, a movable table, etc.) Herein, a loading position signal YL is subtracted from a position command Yr so as to determine a position error ep, and this position error ep is multiplied by the position control gain Kp by the position control portion 701 so that a speed error ev is determined. A speed feedback signal Vf is subtracted from this speed command Vr so as to determine a speed error ev, based on the speed error ev, a torque command (current command) Tr is determined in the speed control portion 702, and based on this torque signal Tr, the motor 703 and the load 704 are driven.
In recent years, demands for higher accuracy and a higher speed have increased in terms of industrial machines and for that purpose, increasing the position loop gain Kp in a full-close system is indispensable. For an increase in the position control gain (or position loop gain), first, it is necessary to increase speed loop gain, however, increasing the gain is difficult due to influence of mechanical resonance characteristics of a ball screw, a nut, etc., of a direct-acting mechanism.
Meanwhile, in the case of a semi-close control system, by applying a widely-known vibration-damping control method by an equivalent rigid model observer (for example, a vibration-damping controller for mechanical vibration as set forth in Japanese Patent Application No. Hei-9-56183) and the like, a mechanical vibration signal that has been sensed by the equivalent rigid model observer is added to a speed command so as to newly become a speed command, whereby making it possible to improve the speed loop gain while suppressing vibration and easily increase loop gain to a value commensurate therewith.
In prior arts, in order to increase the position control gain in full-close control systems, various approaches have been carried out.
In terms of a speed loop of the full-close system, by applying the above-described vibration-damping control, speed gain which is equivalent to that of a semi-close system can be achieved, however, in terms of a position loop, vibration in the control system recurs when the position control gain is increased, therefore, without modification, the upper limit of the position control gain can be obtained only on the order of xc2xd-⅔ of the upper limit value of a semi-close control system. Since a frequency of the recurred vibration is lower than the frequency of vibration that occurs in the speed loop, it cannot be simply considered that a rise in gain in the overall control loop is a cause and causes for the vibration recurrence cannot be clarified (Problem 1).
Apart from a clarification of the causes, priorly, in order to increase the position control gain in full-close systems, various approaches have been carried out.
For example, application of a method (Japanese Unexamined Patent Publication No. Hei-03-110607) can be considered, wherein a signal Xm representing the motor position and a signal XL representing the loading position are added to each other in such a manner as
kxc3x97XL+(1xe2x88x92k)xc3x97Xm(where 0 less than k less than 1)]
so as to become a position feedback signal. When k is approximated to 0, since a feedback content of the loading position becomes less, vibration is reduced, however due to spring characteristics of a drive mechanism, the motor position and loading position signal do not coincide with each other, therefore effects of full-close control are lessened and there is no meaning. In the end, in order to obtain full-close effects, the position control gain is to be increased so as to commensurate with k which has been decreased, therefore, substantial position loop gain remains k=1 and the vibration is not eliminated (Problem 2).
Therefore, a method exists for reducing mechanical vibration within a speed loop by feeding back a torsion angular velocity, which is the difference between a speed of a load and a motor speed to a speed command (Japanese Unexamined Patent Publication Hei-1-251210) or a torque command. When reducing vibration that has recurred in a position loop by this method, since a high-frequency content of the motor speed is contained in the torsion angular velocity, vibration with a high frequency then occurs in the speed loop in turn (there is a possibility that high-frequency vibration occurs due to tuning to low-frequency vibration) and, in the end, only simply applying this method does not become a measure against the above-described vibration recurred in the position loop (Problem 3).
Accordingly, it has been considered that increasing the position control gain in the full-close systems by only using the prior methods is virtually impossible. In order to essentially solve these problems, it is necessary to analyze causes for the recurrence of low-frequency vibration in the position loop.
Therefore, it is an object of the present invention to (by analyzing such causes and suggesting a new control method) provide a position controller for a motor where the position control gain Kp in a full-close control system can be increased to a value equivalent to that of a semi-close control system without causing recurrence of vibration and by increasing the position control gain, highly accurate positioning can be performed in a short time.
In order to achieve the above-mentioned object, according to a first aspect of the invention, a position controller for a motor where a position signal representing the position of a movable table and outputted from a straight-line motion position sensing means attached to a straight-line motion mechanism is used as a position feedback signal comprises:
a differentiating means for differentiating the straight-line motion position signal and outputting a straight-line motion speed signal,
a subtracting means for calculating the difference between a speed command signal and the straight-line motion speed signal, an integrating means for integrating a difference signal outputted from the subtracting means,
a proportional gain means for receiving the output signal from the integrating means, and
an adding means for adding an output signal from the proportional gain means to the speed command signal and outputting a new speed command.
In addition, according to a second aspect of the invention, a position controller for a motor where speed control is performed based on a speed signal obtained by differentiating a rotational position signal of the motor and also position control is performed based on a loading position signal from a position sensor attached to a load driven by the motor compresses:
a differentiating means for differentiating the straight-line motion position signal and outputting a loading speed signal,
a subtracting means for calculating the difference between the loading speed signal and a speed command signal,
a phase adjusting means for performing phase adjustment by inputting a difference signal outputted from the subtracting means into a low-pass filter,
a proportional gain means for receiving an output signal from the phase adjusting means, and
an adding means for adding an output signal from the proportional gain means to the speed command signal and outputting a new speed command.
In addition, according to a third aspect of the invention, the phase adjusting means performs phase adjustment by inputting a difference signal outputted from the subtracting means into a band-pass filter.
In addition, according to a fourth aspect of the invention, a position controller for a motor where speed control is performed based on a speed signal obtained by differentiating a rotational position signal of the motor and also position control is performed based on a loading position signal from a position sensor attached to a load driven by the motor comprises:
an integrating calculation means for integrating a speed command signal,
a subtracting means for calculating the difference between the loading position signal and an integrated signal outputted from the integrating calculation means,
a phase adjusting means for performing phase adjustment by inputting a difference signal outputted from the subtracting means into a band-pass filter,
a proportional gain means for receiving an output signal from the phase adjusting means, and
an adding means for adding an output signal from the proportional gain means to the speed command signal and outputting a new speed command.
According to this position controller for a motor, a difference speed between the loading speed determined by the differentiating means and the speed command is sensed, the difference speed is integrated by the integrating means, and the integrated value is multiplied by gain Kf by the proportional gain means for an addition to the speed signal, wherein said differentiating means, subtracting means for sensing the difference speed, integrating means, and proportional gain means exactly correspond to a vibration-damping controller of a semi-close system where a mechanical vibration signal is sensed and output as a difference speed between an angular velocity of a motor and an estimated angular velocity speed of an equivalent rigid model, thus the gain Kf value of the proportional gain means raises the upper limit of position loop gain Kp without causing recurrence of vibration.
Otherwise, the difference speed between the speed command and loading speed is phase-adjusted by means of the phase adjusting means such as a low-pass filter, a band-pass filter or the like to cancel out a vibration frequency, and the obtained value is multiplied by gain Kfxe2x80x2 by means of the proportional gain means for an addition to the speed command, thus the position loop gain Kp can be increased.